Objectives: Sinonasal inverted papilloma (SNIP) is a rare benign tumor that has potential for malignant transformation, usually into squamous cell carcinoma (SCC). The pre-operative differentiation between SNIP and SNIP-SCC is essential in determining the therapeutic strategy, but it is a challenge, as biopsies may fail to recognize the malignant part of the tumor. Further, a SNIP can also be locally aggressive and thus mimic a malignant tumor. This retrospective study compares the pre-operative differences in computed tomography (CT) and histologic findings between patients with a benign SNIP and those with a SNIP-SCC. 

Methods: Eight patients with SNIP-SCC were selected from the hospital registries of the Department of Otorhinolaryngology, Helsinki University Hospital (Helsinki, Finland). For each case a comparable SNIP case without malignancy was selected. Five histopathologic samples of both the SNIP and SNIP-SCC tumors were retrieved. CT images and the histopathologic samples were re-evaluated by two observers. 

Results: The nasal cavity and ethmoid and maxillary sinuses were the most common sites for both tumor types. The SNIP tumors were mostly unilateral, and the SNIP-SCC tumors were both unilateral and bilateral. Only SNIP-SCC tumors demonstrated bone defects and orbital or intracranial invasion. Dysplastic findings such as dyskeratosis, nuclear atypia, and maturation disturbances were seen only in the SNIP-SCC tumors. 

Conclusions: Bony destruction and invasion of adjacent structures in pre-operative CT images seem to be pathognomonic signs of SNIP-SCC based on this series. To differentiate between SNIP and SNIP-SCC tumors all available pre-operative investigations are warranted.